1. Field of the Invention
This present invention pertains generally to offshore buoyant vessels, platforms, caissons, buoys, spars, or other structures used for supporting offshore oil and gas operations. In particular, the present invention relates to a stable moored offshore terminal, such as would be used for safe handling, staging, and transportation of personnel, supplies, boats, and helicopters.
2. Background Art
Stable buoyant structures for supporting offshore oil and gas operations are known in the art. Offshore production structures, which may be vessels, platforms, caissons, buoys, or spars, for example, each typically include a buoyant hull that supports a superstructure. The hull includes internal compartmentalization for ballasting and storage, and the superstructure provides drilling and production equipment, helipads, crew living quarters, and the like.
In offshore work, on drilling and production platforms for example, a major operating cost arises from the transportation of support and supplies from on-shore facilities. Nearly everything must be carried by boat or by air. Such supply lines are subject to adverse weather and sea states, which have greater effect the farther the supplies must travel. Accordingly, stable floating structures designed to be towed out to sea and moored close to several production platforms within a given field are known in the art. These structures may be used to provide shelter for transportation vessels and to provide support facilities, including storage, maintenance, firefighting, medical, and berthing facilities. Such offshore bases, depots, or terminals may provide a reduction in platform operating costs, as they would allow safer and more cost effective transport of personnel and supplied from the shore, which may be temporarily staged and distributed to local platforms. U.S. Pat. No. 4,984,935 issued to de Oliveira Filho et al. discloses one such floating offshore support structure, which includes a sheltered interior for receiving boats.
A floating structure is subject to environmental forces of wind, waves, ice, tides, and current. These environmental forces result in accelerations, displacements and oscillatory motions of the structure. The response of a floating structure to such environmental forces is affected not only by its hull design and superstructure, but also by its mooring system and any appendages. Accordingly, a floating structure has several design requirements: Adequate reserve buoyancy to safely support the weight of the superstructure and payload, stability under all conditions, and good seakeeping characteristics. With respect to the good seakeeping requirement, the ability to reduce vertical heave is very desirable. Heave motions can create tension variations in mooring systems, which can cause fatigue and failure. Large heave motions increase danger in launching and recovery of small boats and helicopters and loading and offloading stores and personnel.
The seakeeping characteristics of a buoyant structure are influenced by a number of factors, including the waterplane area, the hull profile, and the natural period of motion of the floating structure. It is very desirable that the natural period of the floating structure be either significantly greater than or significantly less than the wave periods of the sea in which the structure is located, so as to decouple substantially the motion of the structure from the wave motion.
Vessel design involves balancing competing factors to arrive at an optimal solution for a given set of factors. Cost, constructability, survivability, utility, and installation concerns are among many considerations in vessel design. Design parameters of the floating structure include the draft, the waterplane area, the draft rate of change, the location of the center of gravity (“CG”), the location of the center of buoyancy (“CB”), the metacentric height (“GM”), the sail area, and the total mass. The total mass includes added mass—i.e., the mass of the water around the hull of the floating structure that is forced to move as the floating structure moves. Appendages connected to the structure hull for increasing added mass are a cost effective way to fine tune structure response and performance characteristics when subjected to the environmental forces.
Several general naval architecture rules apply to the design of an offshore vessel. The waterplane area is directly proportional to induced heave force. A structure that is symmetric about a vertical axis is generally less subject to yaw forces. As the size of the vertical hull profile in the wave zone increases, wave-induced lateral surge forces also increase. A floating structure may be modeled as a spring with a natural period of motion in the heave and surge directions. The natural period of motion in a particular direction is inversely proportional to the stiffness of the structure in that direction. As the total mass (including added mass) of the structure increases, the natural periods of motion of the structure become longer.
One method for providing stability is by mooring the structure with vertical tendons under tension, such as in tension leg platforms. Such platforms are advantageous, because they have the added benefit of being substantially heave restrained. However, tension leg platforms are costly structures and, accordingly, are not feasible for use in all situations.
Self-stability (i.e., stability not dependent on the mooring system) may be achieved by creating a large waterplane area. As the structure pitches and rolls, the center of buoyancy of the submerged hull shifts to provide a righting moment. Although the center of gravity may be above the center of buoyancy, the structure can nevertheless remain stable under relatively large angles of heel. However, the heave seakeeping characteristics of a large waterplane area in the wave zone are generally undesirable.
Inherent self-stability is provided when the center of gravity is located below the center of buoyancy. The combined weight of the superstructure, hull, payload, ballast and other elements may be arranged to lower the center of gravity, but such an arrangement may be difficult to achieve. One method to lower the center of gravity is the addition of fixed ballast below the center of buoyancy to counterbalance the weight of superstructure and payload. Structural fixed ballast such as pig iron, iron ore, and concrete, are placed within or attached to the hull structure. The advantage of such a ballast arrangement is that stability may be achieved without adverse effect on seakeeping performance due to a large waterplane area.
Self-stable structures have the advantage of stability independent of the function of mooring system. Although the heave seakeeping characteristics of self-stabilizing floating structures are generally inferior to those of tendon-based platforms, self-stabilizing structures may nonetheless be preferable in many situations due to higher costs of tendon-based structures.
Prior art floating structures have been developed with a variety of designs for buoyancy, stability, and seakeeping characteristics. An apt discussion of floating structure design considerations and illustrations of several exemplary floating structures are provided in U.S. Pat. No. 6,431,107, issued on Aug. 13, 2002 to Byle and entitled “Tendon-Based Floating Structure” (“Byle”), which is incorporated herein by reference.
Byle discloses various spar buoy designs as examples of inherently stable floating structures in which the center of gravity (“CG”) is disposed below the center of buoyancy (“CB”). Spar buoy hulls are elongated, typically extending more than six hundred feet below the water surface when installed. The longitudinal dimension of the hull must be great enough to provide mass such that the heave natural period is long, thereby reducing wave-induced heave. However, due to the large size of the spar hull, fabrication, transportation and installation costs are increased. It is desirable to provide a structure with integrated superstructure that may be fabricated quayside for reduced costs, yet which still is inherently stable due to a CG located below the CB.
U.S. Pat. No. 6,761,508 issued to Haun on Jul. 13, 2004 and entitled “Satellite Separator Platform (SSP)” (“Haun”), which is incorporated herein by reference, discloses an offshore platform that employs a retractable center column. The center column is raised above the keel level to allow the platform to be pulled through shallow waters en route to a deep water installation site. At the installation site, the center column is lowered to extend below the keel level to improve vessel stability by lowering the CG. The center column also provides pitch damping for the structure. However, the center column adds complexity and cost to the construction of the platform.
Other offshore system hull designs are known in the art. For instance, U.S. Patent Application Publication No. 2009/0126616, published on May 21, 2009 in the name of Srinivasan (“Srinivasan”), shows an octagonal hull structure with sharp corners and steeply sloped sides to cut and break ice for arctic operations of a vessel. Unlike most conventional offshore structures, which are designed for reduced motions, Srinivasan's structure is designed to induce heave, roll, pitch and surge motions to accomplish ice cutting.
U.S. Pat. No. 6,945,736, issued to Smedal et al. on Sep. 20, 2005 and entitled “Offshore Platform for Drilling After or Production of Hydrocarbons” (“Smedal”), discloses a drilling and production platform with a cylindrical hull. The Smedal structure has a CG located above the CB and therefore relies on a large waterplane area for stability, with a concomitant diminished heave seakeeping characteristic. Although, the Smedal structure has a circumferential recess formed about the hull near the keel for pitch and roll damping, the location and profile of such a recess has little effect in dampening heave.
It is believed that none of the offshore structures of prior art, in particular offshore depots or terminals that are arranged to provide shelter to the boats that used for transportation of supplies and personnel to offshore platforms, are characterized by all of the following advantageous attributes: Symmetry of the hull about a vertical axis; the CG located below the CB for inherent stability without the requirement for complex retractable columns or the like, exceptional heave damping characteristics without the requirement for mooring with vertical tendons, and the ability for quayside integration of the superstructure and “right-side-up” transit to the installation site, including the capability for transit through shallow waters. A buoyant offshore depot or terminal possessing all of these characteristic is desirable.
3. Identification of Objects of the Invention
A primary object of the invention is to provide a buoyant offshore depot or terminal characterized by all of the following advantageous attributes: Symmetry of the hull about a vertical axis; the center of gravity located below the center of buoyancy for inherent stability without the requirement for complex retractable columns or the like, exceptional heave damping characteristics without the requirement for mooring with vertical tendons, and a design that provides for quayside integration of the superstructure and “right-side-up” transit to the installation site, including the capability to transit through shallow waters.
Another object of the invention is to provide a buoyant offshore depot or terminal that may be strategically positioned nearby one or more offshore platforms to act as a safe shelter and distribution point for supply boats, helicopters, stores, and personnel.
Another object of the invention is to provide a buoyant offshore depot or terminal with improved pitch, roll and heave resistance.
Another object of the invention is to provide a buoyant offshore depot or terminal that allows fine tuning of the overall system response to meet specific operating requirements and regional environmental conditions.
Another object of the invention is to provide a buoyant offshore depot or terminal that can be constructed without the need for a graving dock, thereby allowing construction in virtually any fabrication yard.
Another object of the invention is to provide a buoyant offshore depot or terminal that is easily scalable.